The use of pyrolytic furnace tubes to crack ethane or other higher number carbon atoms to either ethylene or other hydrocarbonaceous materials has been practiced for at least fifty years. While other methods for producing ethylene are existent in the prior art, this application concerns only the pyrolytic or thermal cracking of ethane.
In 1981, the American Chemical Society conducted a symposium in regard to coke formation on metal surfaces. Some of the presented papers include those of Chapters 1, 2, 4, 5, 6, 7, 9, and 10. In Chapter 1 of that Symposium, Baker and Yates discussed filamentous carbon formation over iron surfaces. Cognizance was taken of the catalytic activity of iron oxide metal on the ethane cracking process at 700.degree. C. In fact, disclosure is made that the pre-treatment of the iron surfaces with steam at 700.degree. C. shows a dramatic increase in the catalytic cracking activity resulting in carbon deposition derivative of the applicable hydrocarbons. This is believed to be the result of the additional formation of FeO compounds. Steam treatment at 800.degree. C. interacts with not only the uppermost layer of the iron in the pyrolytic reaction tubes but also penetrates to an appreciable depth after a period of treatment of about three hours. In either event, the FeO compounds in the iron metal were found to have an extremely high activity with a formation of filamentous deposits of carbon compounds on the surface of iron metal surfaces. This type of coke formation leads to a tube wall metal attrition. It was at least theorized that the reason for high FeO activity is that same is a precursor for a high surface area iron catalyst formed in situ.
In Chapter 2, Brown et al recognize inhibition of coke formation in ethylene steam cracking, i.e. the quantity of coke deposition occurring in the coils and associated downstream heat exchangers can be mitigated, by coating the process vessel walls with a chemical vapor deposition of an alkoxysilane in a steam carrier gas to produce a thin impervious adherent amorphous silica film on the internal surfaces of the steel alloy. From the experimentation performed in regard to that paper, under certain conditions of temperature, steam, concentration, and flow rate, coke formation was found to be not rendered nugatory but was considerably reduced as a derivative of the silica coating.
Chapters 5 and 6 discuss the kinetics and mechanisms for the carbon formation as a result of the pyrolysis of benzene in the presence of hydrogen. Chapters 4, 7, 9, and 10 discuss growth initiation and morphologies of coke deposited on furnace tube walls.